A conventional knife replacement system of a machine center 10 is illustrated in FIG. 1, wherein a shaft 11 has a recess 110 defined in a lower end thereof so that a knife 12 is engaged with the recess 10. A kick member 13 is received in the shaft 11 and clamps the knife 12 by a clamping member 130. A main cylinder 14 exerts a force onto the kick member 130 and an auxiliary cylinder 15 is connected to the system of the main cylinder 14 so as to exert the kick member 13 a steady increment force so force the knife 12 to be disengaged from the recess 110. A knife switch arm (not shown) then pulls the knife 12 from the recess 110 and inserts a new knife 12 in the recess 110. However, it requires a huge force to the knife 12 from the two cylinders 14 and 15 so that the cost of the power is large. The knife 12 has to be maintained still to eliminate the expansion of heat of the knife so that the knife 12 can be easily removed from the recess 110. During the waiting period, the knife switch arm cannot do anything so that the time for replacing a new knife is long. Furthermore, when the cylinders exert a force to the kick member 13, the clamping member 130 is opened slightly so that the knife 12 could drop.